How to determine the pitch for a shed roof rafter.

Submitted by Ray Thornburg on Tue, 11/15/2011 - 13:20

 

blue palmetto home inspection serves charlestonHow to Determine the Pitch for a Shed Roof Rafter

Sometimes it's necessary to cut a shed roof type rafter when you only know the height of the knee-wall or the difference between the two walls and the run.

 

common shed roof pitch math

Difference in height in inches divided by run in feet = pitch 

Conversely pitch times run in feet = Height difference

So in the above example if the height difference is 36" and the run is 12' then 36/12 =3. The pitch is 3 in 12 ( 3/12). Now you can lay out our rafter using 3 as the rise and 12' as the total run. Of course conversely if you multiply the pitch by the run in feet you get the height in inches

Let's try another example. Height difference = 40", Total run 12' 4".

Convert feet to its decimal equivalent by diving by 12. ex. 4"/12 = .33' total run 12.33'

40/12.33 = 3.24 or about a 3 1/4 in 12 pitch.

Once you know the pitch and the total run you can lay the rafter out using a framing square or you can use a calculators trigonometric functions to figure the length.

Note that the birds mouth cut on the rafter will have to be the same at the top and the bottom of the rafter.

If you want to know how to figure the length of a shed roof rafter that actually lays on top of another steeper roof see this page.

 

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Submitted by Ray Thornburg on Tue, 11/15/2011 - 13:20

Comments

Figure roof pitch
Anonymous | Thu, 04/29/2021 - 05:09

My husband and I are adding on to the shed we built last summer. The roof is 16ft. long with a 2 ft difference in height from one side to the other. My question is: We are going to build the roof in 2 8ft sections. How do we figure the slope? Thank you Laura

figure roof pitch
Ray Thornburg | Thu, 05/13/2021 - 12:48

Thank you for your question. I expect there's more details I need to know to truly answer the question. However provide there is no overhang and the hypotenuse of the triangle is 16 feet and the opposite side is 2 foot then the ajacent side would be15.87 feet. I derived this answer from the old middle school pathagorem theeorem A² + B² = C² with C being the hypotenuse of a right triangle.  So conversely C2 - B2 =   A2 with A being the ajacent side. Get the square root of that and you have 15.87 and using the formula above your pitch will be 1.5 in 12. In my book The Roof Framers Field Manual I explain how tho write a script to do this quickly. It's called the Side B Program and it's on page 132. At any rate I hope this helps...

figuring pitch
Anonymous | Wed, 05/15/2013 - 10:16

straightforward. I must have read a dozen articles on how to climb up on the roof and measure 12" and then use a ladder, blah, blah, blah. Your explanation was so easy.

Nice Job!
Anonymous | Fri, 03/22/2013 - 15:07

This is a Straight Forward. No non-sense. "Simple" description of how to do what can be a very complicated task if you don't have the right way to do the cuts. Thank you very much.

It's always nice to hear a kind word
Ray Thornburg | Fri, 03/22/2013 - 15:41

It's always nice to hear a kind word.